1. Technical Field
The present disclosure relates to a radar device.
2. Description of the Related Art
Recent studies have been made on a radar device using a radar transmission signal of a short wavelength including microwave or milliwave with which a high resolution is achieved. Development is required on a radar device (wide-angle radar device) that detects objects (targets) including a pedestrian as well as a vehicle in a wide-angle range in order to improve outdoor safety.
A known example of such a radar device is a pulse radar device that repeatedly emits pulsed waves. A wide-angle pulse radar that detects a vehicle and a pedestrian in a wide-angle range receives a mixture of multiple reflected waves from a target (for example, a vehicle) at a short distance and a target (for example, a pedestrian) at a long distance. This requires (1) a radar transmitter to have a configuration to transmit pulsed waves or pulse-modulated waves having a self correlation characteristic (hereinafter, referred to as a low range sidelobe characteristic) that achieves low range sidelobes, and (2) a radar receiver to have a configuration with a wide reception dynamic range.
Examples of the configuration of the wide-angle radar device include the following two configurations.
The first configuration transmits pulsed waves or modulated waves as radar waves by mechanical or electrical scanning using a directional beam of a narrow angle (beam width of a few degrees), and receives reflected waves using a narrow-angle directional beam. With this configuration, the scanning needs to be performed a large number of times to obtain a high resolution, which leads to a degradation in the performance of following a fast moving target.
The second configuration uses a method (direction of arrival (DOA) estimation) of receiving reflected waves through an array antenna including multiple antennas (antenna elements), and estimating the arrival angle of the reflected waves using a signal processing algorithm based on a reception phase difference corresponding to antenna spacing. This configuration allows the receiving side to estimate the arrival angle even when a frequency of scanning of a transmission beam on the transmitting side is reduced, thereby achieving a shortened scanning time and an improved following performance as compared to the first configuration. Examples of DOA estimation methods include a Fourier transform based on matrix calculation, a Capon method and a linear prediction (LP) method based on inverse matrix calculation, and a multiple signal classification (MUSIC) and an estimation of signal parameters via rotational invariance techniques (ESPRIT) based on eigenvalue calculation.
Disclosed is a radar device (also referred to as a MIMO radar) that includes multiple antennas (array antennas) on the transmitting side as well as the receiving side and performs beam scanning by signal processing using transmitting and receiving array antennas (see Jian Li, Stoica, Petre, “MIMO Radar with Colocated Antennas,” Signal Processing Magazine, IEEE Vol. 24, Issue: 5, pp. 106-114, 2007, for example).
In the MIMO radar, a well-elaborated arrangement of antenna elements in transmitting and receiving array antennas may achieve the formation of virtual receiving array antennas (hereinafter, referred to as a virtual receiving array) whose number at maximum is equal to the product of the number of transmitting antenna elements and the number of receiving antenna elements. This arrangement results in an increased effective aperture length of the array antennas with a small number of elements.
The MIMO radar is applicable not only to one-dimensional scanning in the vertical direction or the horizontal direction but also to two-dimensional beam scanning in the vertical direction and the horizontal direction.
However, when the numbers of antennas on the transmitting and receiving sides are restricted to certain numbers (for example, four transmitting antennas and four receiving antennas) in order to achieve downsizing and low cost of the MIMO radar, a two-dimensional virtual receiving array of the MIMO radar is restricted in terms of aperture lengths in the vertical direction and the horizontal direction.